Ammonia is a widely used chemical with many applications. One specific application is as reductant for selective catalytic reduction (SCR) of NOx in exhaust gas from combustion processes.
For most applications, and in particular in automotive applications, the storage of ammonia in the form of a pressurized liquid in a vessel is too hazardous. Urea is a safe, but an indirect and impractical method for mobile transport of ammonia since it requires to be transformed into ammonia by a process involving thermolysis and hydrolysis ((NH2)2CO+H2O→2 NH3+CO2).
A storage method involving ad- or absorption in a solid can circumvent the safety hazard of anhydrous liquid ammonia and the decomposition of a starting material.
Metal ammine salts are ammonia absorbing and desorbing materials, which can be used as solid storage media for ammonia (see, e.g. WO 2006/012903 A2), which in turn, as mentioned above, may be used as the reductant in selective catalytic reduction to reduce NOx emissions.
Usually, ammonia is released by thermal desorption, e.g. from metal ammine salts, by external heating of a storage container, see e.g. WO 1999/01205 A1. The heating elements may also be placed inside the storage container, see e.g. U.S. Pat. No. 5,161,389 and WO 2006/012903 A2.
In WO 2007/000170 A1 the release of ammonia from the storage material is facilitated by lowering the ammonia pressure in the gas phase.
WO 2008/077652 A2 and DE 10 2007 022 858 A1 disclose systems that use at least two storage materials having different vapor pressures in at least one operation mode.
The present invention seeks to improve the method of desorption of ammonia from an ammonia storage material using a vacuum pump.